1. Field of the Invention
The invention relates to method for manufacturing ceramic materials. In particular, this invention relates to a method for firing ceramics involving separately measuring the interior and surface temperatures and controlling the heating rate of the ceramic material in response to the measured difference.
2. Discussion of the Related Art
Conventional heating used in the manufacturing of ceramic materials typically comprises gas firing or electric resistance heating. Utilization of conventional radiative/convective heating typically results in a temperature differential within the ceramic body, due to the fact that heat is applied only to the surface and it relies mainly on thermal conductivity of the ceramic body, typically poor, to effect the temperature beneath the surface and into the interior of the piece. In other words, conventional heating involves heat transfer that is predominantly achieved by radiation or convection to the surface followed by conduction from the surface into the interior of the ceramic body. If a core-surface temperature differential develops that is too great, cracking and distortion of the ceramic body can occur. Fast firing further exacerbates this problem of poor heat transfer, and ultimately cracking. Additionally, the presence of a core-surface temperature gradient can also result in uneven sintering, specifically surface sintering prior to, and at a faster rate than, interior sintering. As a result, the ceramic body may exhibit non-uniform properties.
Conventional control of the heating used in the manufacturing of ceramic materials typically involved controlling the quantities of heat generated in, or transferred to, the ceramic body by measuring the ambient temperature within an enclosure containing the ceramic body. Based on, and in response to, this ambient temperature measurement, the heat transferred to the ceramic body, is controlled by use of any one of the following heating types: passive or forced convection, and/or conduction. Similarly, radiation heating such as microwave heating, though resulting in the heat being generated within the ceramic body, also involves the use of ambient temperature-based control of the heating. This ambient temperature-based control method of heating the ceramic body, though standard in the ceramic industry, suffers from a number of shortcomings including the following: (1) the mixing of kiln gases may not be uniform enough to accurately predict the ceramic body surface temperatures, thus reducing the effectiveness of the method; (2), many of the chemical reactions that occur within the ceramic body take place at temperatures low enough that ambient gas radiant heat transfer is not a primary means of heat transfer to the ceramic body and to the inside surfaces of the kiln where the kiln ambient temperatures are measured; and, (3) control of the temperature in the kiln space and not control of the temperature of the actual piece does not provide a means for indirectly measuring the stresses exhibited by the ceramic body being heated.
Accordingly it is an object of this invention to provide a method of, efficiently and effectively controlling the heat energy utilized in the heating of ceramics that overcomes the shortcomings of the aforementioned conventional methods of controlling the heating and/or sintering of ceramics.
The firing method of present invention comprises placing the ceramic material in a heating apparatus and subjecting the ceramic material to an amount of heat energy that results in the ceramic article exhibiting a ceramic body core temperature, TC, a ceramic body surface temperature TS, and surface-core temperature differential that is less than or equal to a predetermined maximum temperature differential setpoint. Preferably, the method furthermore involves continuously measuring the ceramic body core temperature, TC, and the surface temperature TS, calculating the measured surface-core differential and adjusting the heat in response to the difference between the measured surface-core differential and the predetermined maximum surface-core differential setpoint.